1. Field of the Invention
The present invention relates to an active distance measuring apparatus suitable for a camera or the like.
2. Related Background Art
Conventionally, as an active distance measuring apparatus for a camera, the one shown in FIG. 84 is known. FIG. 84 is a block diagram of a distance measuring apparatus of the first prior art.
In the distance measuring apparatus shown in FIG. 84, a driver 112 drives an infrared emitting diode (to be referred to as an "IRED" hereinafter) 114 under the control of a CPU 110 to cause the IRED 114 to output infrared light, thereby projecting the infrared light to an object to be subjected to distance measurement. The infrared light reflected by the object is focused on a position sensing device (to be referred to as a "PSD" hereinafter) 116 through an objective (not shown). The PSD 116 outputs two signals I.sub.1 and I.sub.2 in accordance with the position where the reflected infrared light was received. A first signal processing circuit 118 removes the stationary light component contained in the signal I.sub.1 as noise, and a second signal processing circuit 120 removes the stationary light component contained in the signal I.sub.2 as noise.
An arithmetic circuit 132 calculates the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)) on the basis of the signals I.sub.1 and I.sub.2 from which the stationary light components are removed, and outputs an output ratio signal corresponding to the distance to the object. An integration circuit 134 integrates the output ratio signal output from the arithmetic circuit 132 a number of times to improve the S/N ratio. A signal output (to be referred to as an "AF signal" hereinafter) from the integration circuit 134 corresponds to the distance to the object. The CPU 110 performs predetermined calculation on the basis of the AF signal output from the integration circuit 134 to obtain a distance signal and controls a lens driving circuit 136 on the basis of the distance signal to move a lens 138 to an in-focus position.
FIG. 85 is a graph showing the relationship between the AF signal output from the integration circuit 134 and the distance to the object in the first prior art. In this graph, the abscissa indicates the reciprocal (1/L) of a distance L to the object, and the ordinate indicates the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)), i.e., the AF signal. As shown in FIG. 85, up to a certain distance L.sub.4, the output ratio and the reciprocal (1/L) of the distance L have a substantially linear relationship therebetween. As the distance L increases, (1/L decreases), the output ratio decreases. However, at a distance larger than the distance L.sub.4, when the distance L increases, the influence of noise component increases inversely. Letting I.sub.n (I.sub.n .gtoreq.0) be the noise component, the output ratio is given by (I.sub.1 +I.sub.n)/(I.sub.1 +I.sub.n +I.sub.2 +I.sub.n). At a distance larger than the distance L.sub.4, the output ratio varies in the increasing direction. In addition, the noise component I.sub.n is generated at random and the output ratio is therefore unstable depending on specific distance measuring conditions involved. This is because when the distance L increases, the intensity of reflected light reaching the PSD 116 decreases, and the noise component I.sub.n increases relative to the light component. When this phenomenon occurs, the distance L to the object cannot be uniquely determined from the output ratio.
The following techniques are known as distance measuring apparatuses for solving the above problem. FIG. 86 is a block diagram of a distance measuring apparatus of the second prior art. FIG. 86 shows only the arrangement on the light-receiving side. In the distance measuring apparatus shown in FIG. 86, after stationary light components are removed from signals I.sub.1 and I.sub.2 output from a PSD 140 by stationary light removing circuits 142 and 144, respectively, they are input to arithmetic circuits 146 and 148. The arithmetic circuit 146 calculates I.sub.1 /(I.sub.1 +I.sub.2) on the basis of the signals I.sub.1 and I.sub.2 from which the stationary light components are removed, thereby obtaining the output ratio. An integration circuit 150 integrates the output ratio. On the other hand, the arithmetic circuit 148 calculates I.sub.1 +I.sub.2 to obtain the light amount. An integration circuit 152 integrates the light amount. A selection unit 160 selects one of the output ratio and the light amount and obtains the distance to the object on the basis of the output ratio or the light amount. The processing of the selection unit 160 is performed in the CPU.
FIG. 87 is a block diagram of a distance measuring apparatus of the third prior art. FIG. 87 also shows only the arrangement on the light-receiving side. In the distance measuring apparatus shown in FIG. 87, after stationary light components are removed from signals I.sub.1 and I.sub.2 output from a PSD 170 by stationary light removing circuits 172 and 174, respectively, they are input to one terminal of a switch 176. This switch 176 inputs the output from the stationary light removing circuit 172 or 174 to an integration circuit 178 under the control of the CPU. The integration circuit 178 integrates the input signal I.sub.1 or I.sub.2. An arithmetic unit 180 calculates I.sub.1 /(I.sub.1 +I.sub.2) on the basis of the integration result to obtain the output ratio. On the other hand, an arithmetic unit 182 calculates I.sub.1 +I.sub.2 to obtain the light amount. A selection unit 184 selects one of the output ratio and the light amount and obtains the distance to the object on the basis of the output ratio or the light amount. The processing operations of the arithmetic units 180 and 182 and the selection unit 184 are performed in the CPU.
Both the distance measuring apparatuses of the second and third prior arts (FIGS. 86 and 87) obtain a distance L to the object on the basis of the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)) when the distance L is small, or obtain the distance L on the basis of the light amount (I.sub.1 +I.sub.2) when the distance L is large. With this arrangement, the distance L can be uniquely determined.